The invention relates to a thermosetting resin composition useful as a matrix resin for electrical substrate applications. The thermosetting polymers are characterized by exceptional adhesion to copper foil combined with excellent thermal and electrical performance. Moreover, in applications to metal substrates the cured thermosetting compositions exhibit excellent mechanical properties.
The combination of excellent thermal, electrical and mechanical (e.g. laminate bond strength) properties does not inhere in the compositions disclosed in the following U.S. Patents. The composition disclosed in U.S. Pat. No. 5,223,568 to Landi et al. is described to be a thermosetting resin composition for making electrical substrate materials. In general, the composition is formed from 1) a polybutadiene or polyisoprene resin which is a liquid at room temperature and has a molecular weight less than 5,000 and 2) a solid butadiene or isoprene containing polymer capable of cross-linking with the former polybutadiene or polyisoprene resin. The solid butadiene or isoprene containing polymer is a thermoplastic elastomer block copolymer whose function in the composition is to improve the adhesion of the substrate to metal foil. The compositions according to this U.S. patent differ from the compositions of Landi et. al. in that they contain no ethylenically unsaturated monomers with a functionality less than two (2); by the phrase "functionality less than two" is meant that said monomer is characterized as having one double-bond or vinyl group. The patent does not specify ester or urethane chemical functionality(ies). Additionally, according to that U.S. patent, the method of manufacturing requires a high temperature cure condition at a temperature of greater than 250.degree. C. and less than the decomposition temperature of the composition. These cure temperatures exceed the limitations of most commercially available and cost-effective equipment. The cure required by the thermosetting resins described herein, in accordance with the present invention, does not exceed 180.degree. C.
U.S. Pat. No. 5,571,609 to St. Lawrence et al. describes an electrical substrate material similar to U.S. Pat. No. 5,223,568 except for the incorporation of woven glass fabric and particulate filler. The electrical substrates formed from those compositions perform poorly, in the respect that those compositions exhibit low peel bonds or low adhesion of the copper foil to the substrate. The St. Lawrence et al. U.S. Patent provides a composition which yields peel bonds that range from 2.1 to 5 pounds per inch width, (via test method IPC-TM-650 2.4.8 on one ounce per square foot copper foil). Adhesion of the copper foil (peel bond) in this range is marginal for many electrical substrate applications. Low peel bonds can cause subsequently fabricated electrical circuits to exhibit poor reliability and cause failures during high temperature soldering and reflow soldering operations. The peel bonds obtained by the inventive composition of this application typically exceed 10 pounds per inch width, (for one ounce per square foot copper foil).
U.S. Pat. No. 4,241,132 to Pratt et al. discloses a composition suitable for use as a matrix resin in an electrical substrate comprised of polybutadiene along with comonomers, such as styrene and the like. However, unlike the inventive composition of this application, Pratt et al. discloses that the composition is at least 50% by weight and preferably substantially pure polybutadiene. By comparison, as described below, the recited compositions of the instant application differ in that they must contain urethane and/or ester chemical functionality.
It is an object of this invention to provide a thermosetting resin composition that is particularly useful as a matrix resin for an electrical substrate and which can be used on commercially available high-speed processing equipment at temperatures of less than 250.degree. C. To overcome the deficiencies of Landi et al., the thermosetting resin must cure at temperatures substantially less than 250.degree. C. It has been discovered, that the composition recited in this application is compatible with high-speed cost-effective lamination processes, such as those in U.S. Pat. Nos. 4,587,161 and 4,803,022 both of Barrell et al., and does not require cure temperatures above 180.degree. C.
Still another object of this invention is to provide a thermosetting composition that has a combination of both good glass transition temperature (the higher, the better) and good adhesion to the copper foil. It is widely known to those skilled in the art that as a thermosetting polymer increases in glass transition either by increasing the density of cross-links or decreasing the molecular weight between the cross-links, the polymer becomes more brittle. Brittle thermosetting polymers yield electrical substrates that have fabrication problems, (cracking and chipping of the substrate during milling and drilling operations). Also, as the polymer becomes more brittle, (i.e. looses toughness and impact strength), the adhesion of the copper foil decreases. Still another good example of this effect can be seen in epoxy based electrical substrates. As the epoxy composition is changed (by adding epoxy with functionality greater than two) to increase the glass transition temperature from 120.degree. C. to 200.degree. C., the copper adhesion drops from 12 to 8 pounds per inch width. It has been discovered, in accordance with the invention, that a specific combination of ingredients can yield peel bonds over 12 pounds per inch width (for one ounce per square foot copper foil) with very little decrease in glass transition temperature.
It is another object of this invention to provide a thermosetting resin composition with electrical properties suitable for use in high frequency applications. For a polymer dielectric to be suitable for use at broadband high frequencies, the dielectric constant must be stable over decades of frequency and over the environmental extremes of humidity and temperature. The thermosetting compositions and electrical substrates obtained therefrom exhibit these desirable features. The typical moisture absorption of the invented composition is typically less than 0.05% by weight, (weight gain after 24 hours at room temperature in water).